The invention is concerned with a method for purifying liquid of solid and liquid impurities by flotation filtration through which the dirt flocks and liquid impurities 15 in the liquid are separated from the liquid to be purified after which they are led to the top of the liquid to form a sludge bed 16 and from there to continue out to a sludge canal 17 and to a drain, the purified liquid continuing through a sand filter 18 to an isolated space 20 under the filter, through filter nozzles 19 and from there onwards through pipe system 23, for example to a pure water reservoir, which filter is washed in an upstream direction by an opposite water flow.
The best method for purification of drinking water, household water and industrial process water in Finland has proven to be the flotation method. It is equally appropriate for purification of surface water and ground water. It is also very appropriate for purification of both municipal and industrial waste water. The flotation method is very economical due to its small cubic volume requirement, to its many form possibilities, to its effectiveness, to its small power demand and due to the excellent purification results that can be achieved. It can also be very advantageously used to increase the power of old plants and to improve the purification results.
Surface waters in Finland have a very big humus content. When chemical purification is used to remove humus and other impurities very light flocks are produced. Because of this the flotation method is very suitable, since it makes the dirt flocks that are on their way to the surface rise all the way to the surface and furthermore all the flocks rise to the surface.
The process is also adequate for iron bearing ground water, because the fine bubble air used in this process divided in the right way catches the dirt flocks so well that there is no hindrance to their gathering at the surface.
Based on experience it has been proven that no surface or ground draw water exists that cannot be purified by using the flotation process correctly, together with the right chemicals, with the result being good clean water.
The aforementioned results of the flotation process are the same when waste water is being treated because solid particles can be decreased to a minimum.
Two different ways for using the flotation method in liquid purification can be distinguished. One way is to use it as it is for clarification but when optimum purification results are desired, an additive sand filtration is needed.
Although the flotation process is very fast, it has still been found possible to develop a method wherein the flotation process itself and the removal of the sludge which is produced by the process take place in the upper part and the sand filtration takes place in the lower part of the same basin. Because the sand filters will get dirty in due course and will have to be washed, that is, rinsed upstream, and because the flotation sludge bed has to be removed at times, which does not happen at the same time if the plant is being used economically, that will place obstacles in the way of the size of the flotation filters and in the way of the loss of the waste water quantity, particularly when big plants are used. When these are being used this can be a decisive factor.